1. Technical Field
The invention relates generally to switching power conversion devices, and more specifically to high power-density DC-to-AC inverters.
2. Related Art
Growing public concern about the accelerating depletion of the Earth's supply of fossil fuels (e.g., crude oil) and of other contemporary non-renewable energy sources, and energy-related environmental concerns such as pollution and global warming due to emissions, have accelerated the development of alternative energy generation technologies (e.g., fuel cells, photovoltaics, wind power, biomass, tallow trees, etc.) for generating power. Growing concerns about the reliability of local, state and national power distribution infrastructures have promoted interest in on-site power generation technologies, particularly those which are compact enough to be maintained on a private premises while providing enough power to satisfy average power requirements of a home, of a business, or of a cooperating community. Thus, a growing popular demand for technologies for independent production of electric power on the premises of individual homes and businesses, particularly solar-electric arrays and fuel cells, has increased the need for efficient and compact high-power inverters that can convert the Direct Current (DC) typically generated by devices employing such technologies, into the Alternating Current (AC) required to operate most commercially available home appliances and computer equipment. The higher the power-density (i.e., the maximum continuous power rating per unit of volume) of an inverter assembly, the less raw material may be required for its production, and the more convenient its transportation and installation, and concealment on premises can be. Some special applications for high-power inverters, i.e., for powering computers and appliances in individual homes built in remote wilderness settings, or even for powering remote extraterrestrial facilities (e.g., laboratories in orbit, or on the moon, or on other planets), may impose an even stricter necessity for true sine-wave output with a minimization of power inverter volume, or component count, and/or mass per kilowatt of rated capacity.
Pure sine-wave generating inverters generally include semiconductor transistor (e.g., Field Effect Transistor (FET), or insulated-gate bipolar transistor (IGBT)) switches controlled by sine-wave modulated pulse-width-modulated (PWM) signals. The frequency of switching is held constant (e.g., at a frequency higher than the human audio range, e.g., switching frequencies between about 20 kHz–30 kHz) while duty cycles of the alternating switches powering the primary windings are varied to produce an approximately smooth changing (alternating) potential at the secondary winding. The switches interrupt DC currents supplied to a transformer from a DC voltage source (e.g., a battery, solar array) and will generate heat during switching operation. Modified sine-wave inverters also include switches, which are used to generate pulses of alternating voltages of modulated width and fixed voltage. Both pure sine-wave inverters and modified sine-wave inverters may require a low-pass L-C output filter tuned to selectively pass the AC output frequency (e.g., 50 Hz or 60 Hz) to reduce distortion and/or remove high frequency noise. Such filters generally comprise a transformer-output filter inductor and an output filter capacitor coupled to the terminals of the secondary winding of the transformer.
Overheating of the switches often leads to damage to the switches and/or to a failure of the inverter. The heat generated in the switches is wasted energy that reduces the power conversion efficiency of the inverter and tends to limit the maximum power-output, and hence the maximum power-density and/or efficiency, of the inverter. The heat developed in the switches in an inverter generally increases at least proportionally with the frequency of the switching. The related art teaches that it is “difficult to operate a sine-wave-modulated PWM push-pull inverter at switching frequencies higher than approximately 1 kHz”. Mohan, POWER ELECTRONICS, p. 127 (John Wiley & Sons). The academic authorities also teach that PWM push-pull inverters (e.g., producing pure sine-wave output at high efficiency) are generally limited to being “Small Power” (e.g., on the order of 1 KW, e.g., 3 KW or less) inverters, compared to a full-bridge switching inverter having the same size transformer. See. e.g., Constantine Hatziadoniu, http://www.engr.siu.edu/staff/hatz/EE483/A7/sld013.htm
Therefore, although sine-wave-modulated PWM push-pull inverters require half as many switches as full-bridge (e.g., “H” bridge) inverters, they are generally unavailable as compact and high-power true sine-wave output inverters. The size of an inverter (and hence its power-density) is significantly affected by the size of its power transformer, as well as by the number and size of other inverter components such as capacitors, switch banks, heat sinks, and output filter inductors.